The invention involves interrelated structural and processing concepts. Such concepts are generally more clearly explained when related to the prior art where such principles are described as enunciated in the standard technical communications of the art. An example of such a standard communication is the text “Fundamentals of Modern VLSI Devices”, by Y. Taur and T. H. Ning, published by The Cambridge University Press, in 1998 that enunciates the following bipolar transistor structure and terminology.
A bipolar transistor has a base region, a emitter region, and a collector region. The base region directly adjacent the emitter-base junction is the intrinsic base. The rest of the base region is the extrinsic base. The purpose of the extrinsic base is to provide electrical connection to the intrinsic base. The basic operation and device characteristics are determined primarily by the intrinsic base. One of the most important figures of merit of a bipolar transistor is its base transit time. When a bipolar transistor is turned from off to on, minority carriers are injected from the emitter into the intrinsic base and collected at the collector. In an n-p-n bipolar transistor, for example, electrons are injected from the n-type emitter into the p-type quasi-neutral intrinsic base layer. After traversing the quasi-neutral intrinsic base layer, the electrons are collected at the collector. The base transit time is the average time for an electron to traverse the p-type quasi-neutral intrinsic base layer. One important objective in the design of a bipolar transistor is to minimize the base transit time.
One approach in the art used to reduce the base transit time is to engineer the energy bandgap to provide a built-in drift field in the quasi-neutral intrinsic base layer to speed up the transport of minority carriers across it using the principles advanced in the publication “Bipolar Transistor with Graded Bandgap Base” by J. R. Hayes, F. Capasso, A. C. Gossard, R. J. Malik and W. Wiegman, publised in Electronics Letters, Vol. 19, pages 410–411, in 1983 wherein using as an example, the case of a GaAs bipolar transistor. In the example the drift field is accomplished by making the intrinsic base layer out of an alloy of Al and GaAs instead of GaAs alone. The compound semiconductor AlxGa1-xAs has a larger energy bandgap than GaAs. The larger the value of x, the larger the energy bandgap. By adjusting the value of x across the quasi-neutral base layer so that x is larger towards the emitter-base junction and smaller towards base-collector junction, an intrinsic base layer with graded bandgap is obtained
In the structures in the art, the base transit time of a graded-base-bandgap transistor is smaller than that of a transistor where the energy bandgap is not graded in the quasi-neutral base.
In other structures in the art, as described in the publication, “Si/SiGe epitaxial-base transistors—Part I: Materials, physics, and circuits,” by D. L. Harame, J. H. Comfort, J. D. Cressler, E. F. Crabbé, J. Y.-C. Sun, B. S. Meyerson, and T. Tice, in IEEE Trans. Electron Devices 42, pp. 455–468 (1995). The energy bandgap of silicon is larger than that of germanium. A SiGe-base bipolar transistor is a graded-base-bandgap bipolar transistors where the energy bandgap in the quasi-neutral base layer is larger towards the emitter-base junction and smaller towards the collector-base junction. The graded bandgap is achieved by designing the Ge distribution in the quasi-neutral base layer such that the Ge concentration is larger towards the collector end and smaller towards the emitter end.